Many portable electronic devices, such as portable two-way radio devices, are designed with audio output being an important consideration. In a typical audio circuit, the audio amplifier directly drives a speaker. While this is a conventional design approach for most applications, it poses some issues for applications where a device is to be used safely in an atmosphere including volatile components. Such potentially hazardous conditions can be found, for example, in chemical processing operations, mining, petroleum refineries, and so on. In the event such conditions arise, people in such areas may need to communicate, and accordingly, communication devices for use in such conditions need to be designed to be “intrinsically safe” so as to avoid ignition of volatile constituents that may be dispersed in the atmosphere in an area around the device.
The term “intrinsic safety” refers to the design of devices so that that they are intrinsically incapable of causing ignition of volatile atmospheric constituents (e.g. gases, fumes, dust). To accomplish this there are several considerations that must be addressed, and among them are energy storage and the ability of a device component to reach a temperature that can cause ignition. Energy storage refers to, for example, the storage of charge in capacitors and the storage of magnetic flux in inductive components. In considering a given design for intrinsic safety it is assumed that storage components can experience faults (i.e. sudden short or open circuit conditions) which cause near-instant release of the energy stored in the component. Accordingly, the ability of a device to store energy must be such that a sudden release of energy cannot cause a sufficiently energetic event to cause ignition. Similarly, components are examined to determine their thermal response to fault conditions, and whether any individual component can achieve an ignition temperature.
The speaker, driven by the audio amplifier, includes an inductive coil which can store energy, and which also acts as a resistance. In order to avoid an unsafe level of energy storage under fault conditions, the current provided to the speaker coil must be limited. At the same time, the thermal properties of the speaker likewise necessitate a current limit in order to prevent the speaker coil from heating to an unsafe level in the event of a fault. A conventional technique for limiting current provided to the speaker coil is to insert a pair of fuses between the audio amplifier output(s) and the speaker coil. This presents a significant performance issue, however. Under intrinsically safe design conventions, a safety factor is applied to the rating of the fuses. That means it must be assumed that a fuse will allow an excess current by a given safety factor proportion above its rated current. Accordingly, fuses must be selected that limit current substantially below the “safe” current level (based on inductance and thermal considerations). Therefore the maximum current that can be applied to the speaker by the audio amplifier in normal operation is substantially below the determined safe current. Furthermore, the series resistance of the fuses attenuate audio power delivered to the speaker. As a result, the audio power that can be output by the speaker is substantially reduced. Users of intrinsically safe radio devices often complain about the lack of audio output of such devices.
Accordingly, there is a need for an intrinsically safe audio power circuit that avoids the problems associated with series fuses in driving an audio speaker of the portable radio device.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.